Oil sand, such as is mined in the Fort McMurray region of Alberta, generally comprises water-wet sand grains held together by a matrix of viscous bitumen. It lends itself to liberation of the sand grains from the bitumen, preferably by slurrying the oil sand in heated process water, allowing the bitumen to move to the aqueous phase. The oil sand slurry thus formed is further conditioned, for example, in a pipeline, so that the bitumen coalesces and attaches to air bubbles, thereby forming a bitumen froth that can be separated from the sand in a separator such as a gravity separator or cyclonic separator.
The bitumen froth that is produced from oil sands routinely contains about 20-40% by volume dispersed water in which colloidal clay particles are well dispersed. Such an oil-water mixture is very stable and very viscous, having viscosities even higher than the oil alone. Further, the bitumen froth is non-conducting and the density of the bitumen is nearly identical to the density of the water.
Often, however, the formation of the bitumen froth may be at sites far removed from the upgrading facilities. Hence, the bitumen froth may need to be pumped through a relatively large inner diameter pipeline (e.g., 36 inches in diameter) over long distances (often in the order of 35 km) so that the froth can be further upgraded at the existing upgrading facilities. The aforementioned characteristics of the bitumen froth, however, present challenges in transporting such a viscous material through a pipeline. Pumping the bitumen froth under conditions of core-annular flow through the pipeline helps in the transport of bitumen froth. Core-annular flow can be achieved by introducing a less viscous immiscible fluid such as water into the flow of oil, to act as a lubricating layer between the pipe wall and the oil, or it can be achieved naturally as a result of the water already present in the bitumen froth (often referred to as self-lubricating core annular flow).
However, the nature of the bitumen froth and the nature of the flow regime pose problems when trying to accurately measure the flow rate of the bitumen froth through the large diameter pipelines using commercially available flow measuring devices or flow meters. Accurate flow rate measurements are important to provide input for the leak detection system employed when monitoring such pipelines. One of the major problems encountered is that bitumen tends to accumulate on the walls of the flow meter during froth flows, in particular, when the bulk pipeline is being operated at relatively low velocity.
Hence, conventional flow meters such as standard venturi meters fail to give accurate flow readings when testing these larger diameter pipelines used for transporting the bitumen froth. Other conventional meters such as electromagnetic and ultrasonic meters fail to give accurate flow readings due to properties of the bitumen itself or the flow regime. Thus, there is a need to develop a flow meter that can be used to measure the flow of bitumen froth through a larger diameter pipeline.
The present invention is directed towards a venturi type meter that has been designed to minimize the accumulation of bitumen on the walls of the meter and improve the accuracy of flow rate measurements of bitumen froth through a pipeline.